Laser trimmable planar resistors and resistor networks are well known and are commercially available, from the various vendors including Vishay Intertechnology, Inc. (its Ohmtek subsidiary being located in Niagara Falls, N.Y.) in a wide variety of configurations. In the patent literature there are various patents relating to resistors and resistive networks of this general type. See the following U.S. Pat. Nos.: 4,859,981; 4,782,320; 4,785,277; 4,772,774; 4,582,976; 4,565,000; 4,563,564; 4,386,460; 4,375,056; 4,362,737; 4,298,856; 4,146,867; 3,983,528; 3,657,692; and 2,261,667.
In the manufacture of precision resistors great importance is attached to the means of adjusting their ohmic resistance value to a specific, targeted FIGURE and to do so with precision and consistency. It is desirable to do this over as wide a range of resistance values as possible. Such capability permits the manufacture of otherwise-identical resistors to a semi-finished state in large quantities, with attendant economies of scale. Small quantities of these semi-finished resistors can then be adjusted to a final specific resistance value, as required. The greater the range of adjustability, the fewer the number of semi-finished types which need to be "stocked" to cover the entire range of resistance values which may be required in all possible situations.
This need for adjustability over a wide range is especially acute in the case of resistor networks. These consist of a multiplicity of resistive elements, generally of different value, which are usually employed as voltage dividers. In such cases the effectiveness of the network is highly dependent upon all the individual elements possessing nearly identical performance characteristics. Performance characteristic generally refer to the degree of stability exhibited by the resistor under a variety of adverse physical or chemical stresses either externally or internally generated. Uniformity in this respect can be assured if the resistors in a network are all manufactured in a common production lot. They can then be differentiated solely by resistance value in the adjustment operation. The greater the range of adjustability of a given type, the less the dependence upon different production lots with potentially different performance characteristics.